On 06/11/2014 10:10 AM, Daniel P. Berrange wrote:
On Wed, Jun 11, 2014 at 10:00:24AM -0400, John Ferlan wrote:
On 06/11/2014 05:31 AM, Daniel P. Berrange wrote:
On Tue, Jun 10, 2014 at 03:03:55PM -0400, John Ferlan wrote:
Where is the data from the 'unique_id' coming from ? Is this since that Linux makes up, or is it some standard attribute from the hardware ? It would be desirable to document what it is in a way that's not simply refering to Linux sysfs, since sysfs itself is mostly undocumented :-)
Using 'unique_id' was from Osier's initial design - I 'assumed' that his research into this had determined using this was unique enough.
A small amount of searching turns this up:
https://lkml.org/lkml/2011/8/17/274 ... +What: /sys/bus/scsi/devices/host*/scsi_host/host*/unique_id +Date: February, 2003 +KernelVersion: Unknown +Contact: James Bottomley <James.Bottomley@HansenPartnership.com> +Description: + Read only unsigned integer uniquely identifying the SCSI host + within the system. This value is assigned by the low level + driver. + ...
Also from various sources, there's '/linux/include/scsi/scsi_host.h
/* * This is a unique identifier that must be assigned so that we * have some way of identifying each detected host adapter properly * and uniquely. For hosts that do not support more than one card * in the system at one time, this does not need to be set. It is * initialized to 0 in scsi_register. */ unsigned int unique_id;
Hmm, that's all rather fuzzy. I wonder how much "better" this is unique int ID is than the current hostNN numbers. eg is this unique id stable when you plug / unplug iSCSI targets in arbitrary order
Not sure I can answer with authority on the plug/unplug topic; however, as I understand it the hostNN numbers can "change" what they are pointing at after perhaps a reboot or scsi kernel module reload. I'm not an expert in these matters, but as I do understand it hardware is scanned and devices named in the order they are found. Using the PCI address and unique_id in combination provides a bit more of a chance that if the hostNN number changes, then libvirt will/could find the same adapter that was previously used. John